Method of controlling anti-lock brake system for vehicles and method of finding control point in ABS

ABSTRACT

In an anti-lock brake system mounted on a vehicle wherein when the vehicle is braked in an emergency during running, an increase in the road surface friction force or road surface friction coefficient owing to an increase in the brake pressure is detected by a road surface friction force detecting device or road surface friction coefficient detecting device. The optimum control start point associated with an increase in the signal value of the road surface friction force F or road surface friction coefficient μ provided by the road surface friction force detecting device or road surface friction coefficient detecting device is decided by using a decrease in the wheel speed, i.e., by using the wheel speed ω or dω/dt. Thereafter, from the point where the specified value of control based on F or μ, or dF/dt or dμ/dt, the brake pressure is moved from the pressure increasing mode to the pressure retaining or decreasing mode.

FIELD OF THE INVENTION

[0001] A first form of this invention relates to a method of controllingan anti-lock brake system for vehicles which prevents the locking of thewheels upon emergency braking of a vehicle, said method beingcharacterized in that it uses the road surface friction force F or roadsurface friction coefficient μ instead of slip factor used in the priorart, so as to control the system.

[0002] A second form of this invention relates to a method of findingcontrol point in an anti-lock brake system (ABS) for vehicles whichprevents the locking of the wheels upon emergency braking of a vehicle,said method being characterized in that it makes an errorless ABScontrol decision by using a road surface friction force detecting sensoror a road surface friction coefficient detecting sensor.

BACKGROUND OF THE INVENTION Method of Controlling Anti-Lock Brake Systemfor Vehicles

[0003] Generally, conventional anti-lock brake systems for vehicles,e.g., automobiles, automatically control the brake operation such thatthe slip ratio falls in a given range on the basis of the vehicle speedand wheel speed (e.g., Japanese Patent Publication No. 30585/1984 andJapanese, Patent Application Laid-Open Specification No. 61354/1985).The relation between road surface friction coefficient and slip ratiocan vary depending on the road surface conditions and for this reasonsaid systems sometimes fail to provide a maximum brake pressure, inwhich case a minimum brake distance cannot be obtained. Further, sincethe vehicle speed is a value estimated from the wheel speed, there is aproblem on precision in the control of slip ratio. To accurately knowthe vehicle speed, there is a need for a complicated device, such as aground-relative speed sensor (e.g., Japanese Patent ApplicationLaid-Open Specification No. 64861/1988) or a vehicle deceleration sensor(e.g., Japanese Patent Application Laid-Open Specification No.170157/1988). In the device described in Japanese Patent ApplicationLaid-Open Specification No. 25169/1988, the torque of road surfacefriction force (tire torque) acting on the wheel is found by calculationfrom wheel angular acceleration and brake liquid pressure and that valueof the tire torque at which the tire torque starts to decrease duringthe increase of the brake liquid pressure is employed as one of thefactors for deciding the conditions immediately before the locking ofthe wheel. However, in this device, the tire torque is indirectly foundby calculation from the wheel angular acceleration and brake liquidpressure and on account of the presence of uncertain constants such asbrake efficiency and the moment of inertia of the wheel, there is aproblem on precision. Further, since the pneumatic pressure in the tireof the wheel and the distance from the ground to the vehicle vary, thereis also a problem that the ratio between the road surface friction forceand the tire torque is not always maintained at a constant value.

[0004] In order to eliminate the drawback inherent in the conventionaldevice described above, the present applicant has previously proposed,in Japanese Patent Application No. 197809/1989 (Japanese PatentApplication Laid-open Specification No. 220056/1991), an anti-lock brakesystem for vehicles, comprising a strain gauge disposed in the vicinityof the axle, a load surface friction force detecting device having meansfor directly measuring shearing strains in the vicinity of the axle, anda vertical load detecting device, and means whereby in response to anoutput signal from a road surface friction coefficient detecting devicehaving means for arithmetically processing detection signals from saidtwo devices, the brake pressure is increased when the road surfacefriction force or road surface friction coefficient increases withincreasing brake pressure or it is decreased when the road surfacefriction force or road surface friction coefficient decreases despiteincreasing brake pressure, and if the road surface friction force orroad surface friction coefficient decreases with decreasing brakepressure, the brake pressure is increased again, such operations beingrepeated.

[0005] In the case where anti-lock brake control for vehicles iseffected by using said system, it has been found that owing todisturbance sources such as vibrations of the tire and road surfaceduring brake operation and the suspension, the signal value of the roadsurface friction force F or road surface friction coefficient μsometimes fluctuates in a certain range, causing the accurate controlstart point to be mistaken.

Method of Detecting Control Point in ABS

[0006] Generally, conventional anti-lock brake systems for vehicles,e.g., automobiles, automatically control the brake operation such thatthe slip ratio falls in a given range on the basis of the vehicle speedand wheel speed (e.g., Japanese Patent Publication No. 30585/1984 andJapanese Patent Application Laid-Open Specification No. 61354/1985). Therelation between road surface friction coefficient and slip ratio canvary depending on the road surface conditions and for this reason saidsystems sometimes fail to provide a maximum brake pressure, in whichcase a minimum brake distance cannot be obtained. Further, since thevehicle speed is a value estimated from the wheel speed, there is aproblem on precision in the control of slip ratio. To accurately knowthe vehicle speed, there is a need for a complicated device, such as aground-relative speed sensor (e.g., Japanese Patent ApplicationLaid-Open Specification No. 64861/1988) or a vehicle deceleration sensor(e.g., Japanese Patent Application Laid-Open Specification No.170157/1988). In the device described in Japanese Patent ApplicationLaid-Open Specification No. 25169/1988, the torque of road surfacefriction force (tire torque) acting on the wheel is found by calculationfrom wheel angular acceleration and brake liquid pressure and that valueof the tire torque at which the tire torque starts to decrease duringthe increase of the brake liquid pressure is employed as one of thefactors for deciding the conditions immediately before the locking ofthe wheel. However, in this device, the tire torque is indirectly foundby calculation from the wheel angular acceleration and brake liquidpressure and on account of the presence of uncertain constants such asbrake efficiency and the moment of inertia of the wheel, there is aproblem on precision. Further, since the pneumatic pressure in the tireof the wheel and the distance from the ground to the vehicle vary, thereis also a problem that the ratio between the road surface friction forceand the tire torque is not always maintained at a constant value.

[0007] In order to eliminate the drawback inherent in the conventionaldevice described above, the present applicant has previously proposed,in Japanese Patent Application No. 197809/1989 (Japanese PatentApplication Laid-open Specification No. 220056/1991), an anti-lock brakesystem for vehicles, comprising a strain gauge disposed in the vicinityof the axle, a load surface friction force detecting device having meansfor directly measuring shearing strains in the vicinity of the axle, anda vertical load detecting device, and means whereby in response to anoutput signal from a road surface friction coefficient detecting devicehaving means for arithmetically processing detection signals from saidtwo devices, the brake pressure is increased when the road surfacefriction force or road surface friction coefficient increases withincreasing brake pressure or it is decreased when the road surfacefriction force or road surface friction coefficient decreases despiteincreasing brake pressure, and if the road surface friction force orroad surface friction coefficient decreases with decreasing brakepressure, the brake pressure is increased again, such operations beingrepeated.

[0008] In the case where anti-lock brake control for vehicles iseffected by using said system, it has been found that owing tocrosstalks such as brake torque contained in sensor signals, theaccurate control start point can often be mistaken.

SUMMARY OF THE INVENTION Method of Controlling Anti-Lock Brake Systemfor Vehicles

[0009] In view of the problem inherent in said anti-lock brake systemfor vehicles according to the prior art described above, the presentinvention has for its object the provision of a method of controlling ananti-lock brake system for vehicles, said method being improved toeffect stabilized control by eliminating disturbance sources in thevicinity of the optimum control start point concomitant with theincrease of the signal value of the road surface friction force F orroad surface friction coefficient μ or by confining disturbance sourcesassociated with the optimum control start point in a range of givenwidth, said method preventing the brake pressure from beingunnecessarily decreased.

[0010] The invention described in claim 1 is a method of controlling ananti-lock brake system for vehicles wherein when a vehicle having ananti-lock brake system mounted thereon has the emergency brake appliedthereto, a change in the road surface friction force or road surfacefriction coefficient due to the increased brake pressure is detected bya road surface friction force detecting device or a road surfacefriction coefficient detecting device,

[0011] said method being characterized in that to decide the optimumcontrol start point concomitant with the increase of the signal value ofthe road surface friction force F or road surface friction coefficient μdetected by the road surface friction force detecting device or roadsurface friction coefficient detecting device, use is made of means foreliminating disturbance sources which impede such decision.

[0012] According to the invention described in claim 1, eliminatingdisturbance sources in the vicinity of the optimum control start pointconcomitant with the increase of the signal value of the road surfacefriction force F or road surface friction coefficient μ or confiningdisturbance sources associated with the optimum control start point in arange of given width ensures that there is no unnecessary decrease inbrake pressure and that efficient and stabilized control is effected.

[0013] The invention as described in claim 2 is a method of controllingan anti-lock brake system for vehicles, wherein when a vehicle having ananti-lock brake system mounted thereon has the emergency brake appliedthereto, a change in the road surface friction force or road surfacefriction coefficient due to the increased brake pressure is detected bya road surface friction force detecting device or a road surfacefriction coefficient detecting device,

[0014] said method being characterized in that the optimum control startpoint concomitant with the increase of the signal value of the roadsurface friction force F or road surface friction coefficient μ due tothe increased brake pressure detected by the road surface friction forcedetecting device or road surface friction coefficient detecting deviceis decided by a drop in the wheel speed, that is, by using the wheelspeed ω or dω/dt, and then from the point where the specified value ofcontrol by F or μ or by dF/dt or dμ/dt is reached, the brake pressure isshifted from the pressure increasing mode to the pressure retaining ordecreasing mode, so as to eliminate disturbance sources.

[0015] According to the invention described in claim 2, a drop in thewheel speed is decided by the wheel speed ω or dω/dt and the optimumcontrol start point concomitant with the increase of the signal value ofthe road surface friction force F or road surface friction coefficient μis specified, thereby providing the optimum control start point freefrom the influences of disturbance sources during ABS control.

[0016] The invention described in claim 3 is a method of controlling ananti-lock brake system for vehicles wherein when a vehicle having ananti-lock brake system mounted thereon has the emergency brake appliedthereto, a change in the road surface friction force or road surfacefriction coefficient due to the increased brake pressure is detected bya road surface friction force detecting device or a road surfacefriction coefficient detecting device,

[0017] said method being characterized in that the optimum control startpoint concomitant with the increase of the signal value of the roadsurface friction force F or road surface friction coefficient μ due tothe increased brake pressure detected by the road surface friction forcedetecting device or road surface friction coefficient detecting deviceis decided by a drop in the vehicle acceleration decided by dV/dt usingan acceleration sensor, and then from the point where the specifiedvalue of control by F or μ or by dF/dt or dμ/dt is reached, the brakepressure is shifted from the pressure increasing mode to the pressureretaining or decreasing mode, so as to eliminate disturbance sources.

[0018] According to the invention described in claim 3, a drop in thevehicle acceleration is decided by dV/dt using an acceleration sensorand then the optimum control start point concomitant with the increaseof the road surface friction force F or road surface frictioncoefficient μ is designated, thereby providing the optimum control startpoint free from the influences of disturbance sources during ABScontrol.

[0019] The invention described in claim 4 is a method of controlling ananti-lock brake system for vehicles wherein when a vehicle having ananti-lock brake system mounted thereon has the emergency brake appliedthereto, a change in the road surface friction force or road surfacefriction coefficient due to the increased brake pressure is detected bya road surface friction force detecting device or a road surfacefriction coefficient detecting device,

[0020] said method being characterized in that the optimum control startpoint concomitant with the increase of the signal value of the roadsurface friction force F or road surface friction coefficient μ detectedby the road surface friction force detecting device or road surfacefriction coefficient detecting device is decided by both the wheel speedω or dω/dt and dV/dt, and then from the point where the specified valueof control by F or μ or by dF/dt or dμ/dt is reached, the brake pressureis shifted from the pressure increasing mode to the pressure retainingor decreasing mode, so as to eliminate disturbance sources.

[0021] According to the invention described in claim 4, a drop in thewheel speed is decided by the wheel speed ω or dω/dt and a drop in thevehicle acceleration is decided by dV/dt using an acceleration sensorand then the optimum control start point concomitant with the increaseof the road surface friction force F or road surface frictioncoefficient μ is designated, thereby providing the optimum control startpoint free from the influences of disturbance sources during ABScontrol.

[0022] According to the present invention described in claims 2 through4, a drop in the wheel speed during brake operation is detected by thespecified value of ω or by dω/dt so as to provide a control start pointor a drop in the vehicle speed is detected by dV/dt so as to provide acontrol start point or detected by both to be decided as the controlstart point; therefore, the optimum control start point is obtainedwithout being influenced by disturbance sources such as vibrationsbetween the tire and road surface during brake operation and thesuspension, thus enabling accurate control to be effected using F or μ.

[0023] The invention described in claim 5 is a method of controlling ananti-lock brake system for vehicles as set forth in claims 1 through 4,characterized in that in the pressure decreasing mode subsequent to theretaining mode or after moving to the pressure decreasing mode, thebrake pressure is shifted from the pressure decreasing mode to thepressure increasing mode by a pressure decreasing threshold value, a setvalue of a specified value of elapsed time or a decision circuit or by acombination of two or more of such factors, so as to eliminatedisturbance sources.

[0024] According to the invention described in claim 5, any one of thecontrol methods described in claims 1 through 4 is used to shift thebrake pressure from the pressure increasing mode to the retaining modeor pressure decreasing mode and then the brake pressure is shifted fromthe pressure decreasing mode to the pressure increasing mode by apressure decreasing threshold value, a set value of a specified value ofelapsed time or a decision circuit or by a combination of two or more ofsuch factors, so as to eliminate disturbance sources, thus effecting ABScontrol.

[0025] The invention described in claim 6 is a method of controlling ananti-lock brake system for vehicles, characterized in that during travelof a vehicle having an anti-lock brake system, the first brake pressurecontrol is effected by using any of the control methods described inclaims 1 through 4, and then at the point of time when the controlmethod described in claim 5 is completed, the control is continuouslyrepetitively effected by using any of the control methods described inclaims 1 through 4 or the control method described in claim 5 until thevehicle stops or in and after the second time the control iscontinuously repetitively effected by using F or μ, or dF/dt or dμ/dtalone and the control method described in claim 5 until the vehiclestops, thus eliminating disturbance sources.

[0026] According to the invention described in claim 6, the first brakepressure control is effected by using any of the control methodsdescribed in claims 1 through 4, and then after the brake pressure hasbeen shifted from the pressure increasing mode to the retaining orpressure decreasing mode, the brake pressure is shifted from thepressure decreasing mode to the pressure increasing mode by a pressuredecreasing threshold value, a set value of a specified value of elapsedtime or a decision circuit or by a combination of two or more of suchfactors, whereupon said control is continuously repetitively effecteduntil the vehicle stops or after the second time the control iscontinuously repetitively effected using F or μ, or dF/dt or dμ/dt aloneand the control method described in claim 5 until the vehicle stops.

[0027] According to the invention described in claims 5 and 6, after anyof the operations described in claims 2 through 4 has been performed andthe brake pressure has been shifted to the pressure decreasing mode, thecontrol is continuously repetitively effected in which the brakepressure is shifted from the pressure decreasing mode to the pressureincreasing mode by a pressure decreasing threshold value, a set value ofa specified value of elapsed time or a decision circuit or by acombination of two of such factors, or in and after the second time thecontrol is continuously repetitively effected using F or μ or dF/dt ordμ/dt alone and the aforesaid control method; therefore, the shift ofthe brake pressure from the pressure decreasing mode to the pressureincreasing mode is continuously repetitively effected until the vehiclestops, so that disturbance in the vicinity of the F or μ brake optimumvalue can be eliminated.

[0028] The invention described in claim 7 is a method of controlling ananti-lock brake system for vehicles wherein when a vehicle having ananti-lock brake system mounted thereon has the emergency brake appliedthereto, a change in the road surface friction force or road surfacefriction coefficient due to the increased brake pressure is detected bya road surface friction force detecting device or a road surfacefriction coefficient detecting device,

[0029] said method being characterized in that after the maximum valueof the signal value of the road surface friction force F or road surfacefriction coefficient μ detected by the road surface friction detectingdevice or road surface friction coefficient detecting device has beenascertained, fixed lower limits are provided for the approximate maximumF value or approximate maximum μ value and the maximum F value ormaximum μ value and the brake pressure is controlled such that F or μ isstably retained in the range, thus eliminating disturbance sources.

[0030] According to the invention described in claim 7, the method isnot influenced by fluctuations in the F or μ value caused by disturbanceand the brake pressure maintains approximately the maximum F value ormaximum μ value without any unnecessary decrease in brake pressure, sothat efficient and stabilized control can be effected.

[0031] The invention described in claim 8 is a method of controlling ananti-lock brake system for vehicles wherein when a vehicle having ananti-lock brake system mounted thereon has the emergency brake appliedthereto, a change in the road surface friction force or road surfacefriction coefficient due to the increased brake pressure is detected bya road surface friction force detecting device or a road surfacefriction coefficient detecting device,

[0032] said method being characterized in that after the maximum valueof the signal value of the road surface friction force F or road surfacefriction coefficient μ detected by the road surface friction detectingdevice or road surface friction coefficient detecting device has beenascertained, fixed lower limits are provided for the approximate maximumF value or approximate maximum μ value and the maximum F value ormaximum μ value and the brake pressure is controlled by being increasedor decreased in the range in which F or μ is retained, thus eliminatingdisturbance sources.

[0033] According to the invention described in claim 8, the brakepressure is retained between the maximum F value or maximum μ value andthe F value or μ value which is in the disturbance range, and efficientand stabilized control is effected without any unnecessary decrease inbrake pressure.

[0034] According to the invention described in claims 7 and 8, incontrast to the fact that in the conventional control method, in orderto prevent the locking of the wheel in the vicinity of the limit ofbrake force in the possession of a road surface, it has been necessaryto once decrease the brake pressure to decrease the brake force on theroad surface, the present invention makes it unnecessary to decrease thepressure, enabling the brake pressure to be retained in the vicinity ofthe maximum value of brake force in the possession of the road surface.Thereby, the brake distance for the emergency brake can be decreased,improving the safety of the vehicle.

[0035] The invention described in claim 9 is a method of controlling ananti-lock brake system for vehicles, characterized in that in claims 7and 8, variations in F or μ with respect to brake pressure are monitoredand when the upper limit of brake pressure in a preset control range isreached, if F or μ exceeds the previous maximum F or maximum μ, thebrake pressure is further increased to ascertain the maximum g again,whereafter the above-mentioned brake pressure control is effected, thuseliminating disturbance sources.

[0036] According to the invention described in claim 9, during controlaccording to the control method described in claims 7 and 8, if the Fvalue or μ value obtained from the road surface increases, that is, whenthe vehicle moves from a slippery road surface to a less slippery roadsurface, the brake force obtained from the road surface is efficientlyused to shorten the brake distance.

[0037] According to the invention described in claim 9, in contrast tothe fact the conventional control method has no means for knowing thevehicle having moved to a road surface which provides a higher roadsurface brake force during anti-lock brake operation, the change of theroad surface can be detected with good responsibility to retain theoptimum brake pressure. Thereby, the road surface brake force which canbe primarily acquired is effectively used to shorten, thus improving thesafety of the vehicle.

[0038] The invention described in claim 10 is a method of controlling ananti-lock brake system for vehicles, characterized in that in claims 7and 8, variations in F or μ with respect to brake pressure are monitoredand if the F value or μ value decreases with respect to the brakepressure in a preset brake pressure control range, the brake pressure israpidly decreased to ascertain the maximum μ again, thus effectingcontrol to eliminate disturbance sources.

[0039] According to the invention described in claim 10, during controlaccording to the control method described in claims 7 and 8, if the Fvalue or μ value obtained from the road surface decreases, that is, whenthe vehicle moves a more slippery road surface, the brake force israpidly decreased to avoid locking and the optimum brake pressurecontrol on the road surface in question is effected.

[0040] According to the invention described in claim 10, in contrast tothe fact that the conventional control method makes the detection onlywhen the wheel actually starts to be locked, the detection is made inthe stage where the wheel is supposed to start to be locked, making itpossible to start adjusting the brake pressure in the early period, thuseliminating the need for adjusting excessive brake pressure resultingfrom delayed decision, so that the optimum brake pressure control can beeffected.

[0041] The invention described in claim 11 is a method of controlling ananti-lock brake system for vehicles, characterized in that in the casewhere the maximum value of the signal value of the road surface frictionforce F or road surface friction coefficient μ detected by the roadsurface friction force detecting device or road surface frictioncoefficient detecting device can hardly be ascertained, quasi-F orquasi-μ is found between the maximum F value or maximum μ value and theminimum F value or minimum μ value within a given period of time, andthe brake pressure control is effected corresponding to such quasi-F orquasi-μ, thus eliminating disturbance sources.

[0042] According to the invention described in claim 11, stabilizedbrake pressure control can be effected even on a road surface whichcauses vigorous vibrations, such as an undulating road surface.

[0043] The invention described in claim 12 is a method of controlling ananti-lock brake system for vehicles, characterized in that in claim 11,in the case where the value between the maximum F value or maximum μvalue and the minimum F value or minimum μ value within a given periodof time varies beyond the allowable range, the individual values aremeasured again within a given period of time to newly find quasi-F orquasi-μ and the brake pressure control is effected corresponding to suchquasi-F or quasi-μ, thus eliminating disturbance sources.

[0044] According to the invention described in claim 12, optimum brakepressure control can be effected even on a road surface which causesvigorous vibrations, such as an undulating road surface.

[0045] The invention described in claim 13 is a method of controlling ananti-lock brake system for vehicles, characterized in that in claims 11and 12, in the case where the maximum μ can be ascertained, control iseffected according to the control methods described in claims 7 through10, thus eliminating disturbance sources.

[0046] According to the invention described in claim 13, effectivestabilized anti-lock brake control can be effected even on a roadsurface which causes vigorous vibrations, such as an undulating roadsurface.

[0047] According to the invention described in claims 11 through 13,stabilized anti-lock brake control can be effected even on a roadsurface where the detected value of F or μ are high in variation, suchas an undulating road surface.

Method of Detecting Control Point in ABS

[0048] In view of the above problems inherent in the anti-lock brakesystems for vehicles according to the prior art the invention has forits object the provision of a method of detecting the control pointwhich enables normal ABS control even if a sensor liable to crosstalk orthe like is used.

[0049] The invention described in claim 14 is a method of detecting thecontrol point in an ABS having a stress sensor which provides an outputproportional to the road surface friction F or road surface frictioncoefficient μ having mixed therein a crosstalk component, such as braketorque T, and to the brake torque T, said method being characterized inthat it uses adjusting means for making adjustment from the rise of thebrake start such that detected signals of F or μ and T are adjusted inratio or made equal in value, and decision means for deciding thecontrol point by a change in said ratio or in adjustment coefficient,wherein the timing of the control point is detected by the size of achange in the ration of detected signals of F or μ and T or inadjustment coefficient.

[0050] According to the invention described in claim 14, in order todetect the timing of the control point, the detected signals of F or μand T are adjusted in ratio or made equal in value from the rise of thebrake start and the control point is calculated by the size of thechange in ratio or in adjustment coefficient; thus, the control pointwhich is not influenced by crosstalk or the like can be obtained.

[0051] The invention described in claim 15 is a method of detecting thecontrol point in an ABS, characterized in that in the decision means ofclaim 14, the point of time when the ratio of F or μ and T or adjustmentcoefficient substantially stops changing is decided to be the controltiming. Thereby, the control point which is not influenced by crosstalkor the like can be obtained.

[0052] The invention described in claim 16 is a method of detecting thecontrol point in an ABS, characterized in that in the decision means ofclaim 14, the point of time when the ratio of F or μ and T or theadjustment coefficient, during brake pressure decreasing control,becomes substantially equal to the value obtained upon completion of thepreceding brake pressure decreasing control is decided as the completionof the brake pressure decreasing control, said point of time beingdecided to be the optimum control timing for ABS. Thereby, the controlpoint which is not influenced by crosstalk or the like can be obtained.

[0053] The invention described in claim 17 is a method of detecting thecontrol point in an ABS, characterized in that in the decision means ofclaim 14, the point of time when the ratio of F or μ and T or theadjustment coefficient during the brake pressure decreasing control,becomes above the value obtained during the first brake pressuredecreasing control is decided to be the completion of the brake pressuredecreasing control, said point of time being decided to be the optimumcontrol timing for the ABS. Thereby, the control point which is notinfluenced by crosstalk or the like can be obtained.

[0054] The invention described in claim 18 is a method of detecting thecontrol point in an ABS, characterized in that in the decision means ofclaim 14, the point of time when the ratio of F or μ and T or theadjustment coefficient, during the brake pressure decreasing control,starts to increase is decided to be the completion of the brake pressuredecreasing control, said point of time being decided to be the optimumcontrol timing for the ABS. Thereby, the control point which is notinfluenced by crosstalk or the like can be obtained.

[0055] The invention described in claim 19 is a method of detecting thecontrol point in an ABS, characterized in that in the decision means ofclaim 14, the point of time when the ratio of F or μ and T or theadjustment coefficient, except during the brake Pressure decreasingcontrol, becomes just above the value obtained during the first brakepressure decreasing control is decided as the start of brake pressuredecreasing control, said point of time being decided to be the optimumcontrol timing for the ABS. Thereby, the control point which is notinfluenced by crosstalk or the like can be obtained.

[0056] The invention described in claim 20 is a method of detecting thecontrol point in an ABS, characterized in that in the decision means ofclaim 14, the point of time when the ratio of F or μ and T or theadjustment coefficient, during the brake pressure retaining control,becomes greater than the value obtained during the first brake pressuredecreasing control is decided to be the start of brake pressurizationcontrol, said point being decided to be the optimum control timing forthe ABS. Thereby, the control point which is not influenced by crosstalkor the like can be obtained.

[0057] The invention described in claim 21 is a method of detecting thecontrol point in an ABS, characterized in that in the decision means ofclaim 14, the point of time when the ratio of F or μ and T or theadjustment coefficient, during the brake pressure retaining control,becomes smaller than the value obtained during the first brake pressuredecreasing control and becomes further smaller is decided to be thestart of brake pressure decreasing control, said point being decided tobe the optimum control timing for the ABS. Thereby, the control pointwhich is not influenced by crosstalk or the like can be obtained.

[0058] The invention described in claim 22 is a method of detecting thecontrol point in an ABS, characterized in that in the decision means ofclaim 14, the point of time when the ratio of F or μ and T or theadjustment coefficient, during the brake pressure retaining control,becomes smaller than the value obtained during the first brake pressuredecreasing control and is stabilized is decided to be continuation ofpressure retention or the start of control of gentle increase ofpressure, said point of time being decided to be the optimum controltiming for the ABS. Thereby, the control point which is not influencedby crosstalk or the like can be obtained.

[0059] According to the invention described in claims 14 through 22, todetect the timing for control point, the detected signal of F or μ and Tare adjusted in ratio or made equal in value from the rise of brakestart and the control point can be calculated from the size of thechange in the ratio or adjustment coefficient to provide the controlpoint which is not influenced by crosstalk or the like; therefore, usingthe detected value as such which has a crosstalk component such as braketorque which is difficult to eliminate when detecting the road surfacefriction force F or road surface friction coefficient μ, it is possibleto find the accurate control point, and in ABS control in which safetyis the most important factor, the probability of erroneous decision canbe minimized.

[0060] The invention described in claim 23 is a method of detecting thecontrol point in an ABS having a stress sensor which provides an outputproportional to the road surface friction F or road surface frictioncoefficient μ having mixed therein a crosstalk component, such as braketorque T, and to the brake torque T, said method being characterized inthat it uses adjusting means for adjusting the ratio of detected signalsof F or μ and T, and decision means for deciding the control point by achange in F−T value from its value after adjustment, wherein the timingfor the control point is detected by said change in the F−T value.

[0061] According to the invention described in claim 23, in order todetect the timing for the control point, and T, the timing for thecontrol point is calculated and decided by a change in the F−T valueduring brake control and thus the crosstalk component T, which isoriginally a noise, is positively utilized to provide the optimumcontrol point.

[0062] The invention described in claim 24 is a method of detecting thecontrol point in an ABS having a stress sensor which provides an outputproportional to the road surface friction force F or road surfacefriction coefficient μ and the brake torque T, said method beingcharacterized in that the timing for the control point is detected by achange in the F−T value during brake control.

[0063] According to the invention described in claim 24, in order todetect the timing for the control point, and T, the timing for thecontrol point is calculated and decided by a change in F−T value duringbrake control and thus the crosstalk component T, which is originally anoise, is positively utilized to provide the optimum control point.

[0064] The invention described in claim 25 is a method of detecting thecontrol point in an ABS, characterized in that in the decision means inclaims 23 and 24, a threshold value for the F−T value is provided todecide the control point in control.

[0065] According to the invention described in claim 25, to detect thetiming for the control point, a threshold value for the F−T value isprovided during brake control to decide the timing in control or decidethe timing for the control of F, N, ΔF, ΔN, thereby eliminating theinfluences of noise.

[0066] The invention described in claim 26 is a method of detecting thecontrol point in an ABS, characterized in that in the threshold value ofclaim 25, a change in the F−T threshold value is corrected as the roadsurface friction coefficient changes, thereby detecting the timing forthe control point.

[0067] According to the invention described in claim 26, in order todetect the timing for the control point, a threshold value is used forthe F−T value to correct the threshold value for the F−T value as theroad surface friction coefficient changes, so as to decide the timingfor the control point; thus, as the vehicle speed changes from high tolow value, the threshold value is decreased according as the roadsurface friction coefficient decreases, thereby providing the optimumcontrol point.

[0068] In the invention described in claims 23 through 26, the T valuewhich is originally a noise inherent in a sensor used in the inventionis used in the form of F−T, whereby without using a gear-like wheelspeed sensor used in conventional ABSs, hybrid control by a singlesensor can be effected which uses both slip ratio control with higherresponse and control decision based on F or μ according to theinvention.

[0069] The present invention will now be described in more detail withreference to embodiments thereof shown in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0070]FIG. 1 is a block diagram showing an example of a hard arrangementfor a control method for an anti-lock brake system for vehiclesaccording to the present invention described in claim 2;

[0071]FIG. 2 is a block diagram showing an example of a hard arrangementfor a control method for an anti-lock brake system for vehiclesaccording to the present invention described in claim 3;

[0072]FIG. 3 is a block diagram showing an example of a hard arrangementfor a control method for anti-lock brake system for vehicles accordingto the present invention described in claim 4;

[0073]FIG. 4 is a block diagram showing an example of a hard arrangementfor a control method for an anti-lock brake system for vehiclesaccording to the present invention described in claims 5 and 6;

[0074]FIG. 5 is a flowchart showing a main routine processing of acontrol device shown in FIG. 4;

[0075]FIG. 6 is a flowchart showing a brake liquid pressure decreaseprocessing routine shown in FIG. 5;

[0076]FIG. 7 is a flowchart showing a brake liquid pressure re-increaseprocessing routine shown in FIG. 5;

[0077]FIG. 8 is a flowchart showing an interruption with respect to themain routine processing shown in FIG. 5;

[0078]FIG. 9 is a flowchart showing a control method for an anti-lockbrake system for vehicles according to the present invention describedin claim 7;

[0079]FIG. 10 is a flowchart showing a control method for an anti-lockbrake system for vehicles according to the present invention describedin claim 8;

[0080]FIG. 11 is a flowchart showing a control method for an anti-lockbrake system for vehicles according to the present invention describedin claim 9;

[0081]FIG. 12 is a flowchart showing a control method for an anti-lockbrake system for vehicles according to the present invention describedin claim 10;

[0082]FIG. 13 is a flowchart showing a control method on the basis ofthe road surface friction force F according to the present inventiondescribed in claim 7;

[0083]FIG. 14 is a functional system diagram in the present invention;and

[0084]FIG. 15 is an output trend graph of road surface friction force,brake torque and brake oil pressure during hard braking.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Method of Controlling Anti-LockBrake System for Vehicles

[0085]FIG. 1 is a block diagram showing an example of a hard arrangementfor a control method for an anti-lock brake system for vehicles usingthe road surface friction coefficient according to the present inventiondescribed in claim 2. In the figure, the character A denotes an ω, {dotover (ω)} detecting device; 1 denotes a road surface frictioncoefficient μ detecting device; 2 denotes a brake pedal stepping-onforce sensor; 3 denotes a control device; 4 denotes a brake liquidpressure generating device; 5 denotes a brake device; and 6 denotes abrake liquid pressure detecting device. The control device 3 is composedof electronic circuits including a microprocessor, a memory and aninput/output interface, and is adapted to operates according to aprogram written in the memory in advance.

[0086] In the case where anti-brake system (ABS) control is effectedusing a road surface friction coefficient μ (μ sensor), the optimumcontrol start point is often misunderstood owing to disturbance sourcessuch as vibrations produced in the tire and road surface during braking,and the suspension. To eliminate this disturbance in the vicinity of theμ control optimum value to obtain the optimum control start point, inthis example, the drop in the wheel speed owing to braking is detectedby a predetermined value of ω or μ defined by$\mu = \frac{{\omega}/{t}}{N}$

[0087] and using this as the control start point, the known ABS controlmethod is effected using μ.

[0088] As for the specified value of ω, in the case where the value atthe start of measurement is ω1 and the value at the specified time isω2, the specified value is expressed by$\frac{{\omega 1} - {\omega 2}}{\omega 1} \geqq {0.03\quad {to}\quad 0.2}$

[0089] Expressed using dω/dt, from the equation of wheel motion,

I dω/dt=μ·N·r−T _(B)

[0090] In the above equation, I is the wheel inertia, N is the wheelload, r is the wheel radius, T_(B) is the brake torque, and μ is thefriction coefficient.

[0091] In addition, the brake torque T_(B) is found from

T _(B)=2·μ_(P) ·B·A·P _(w).

[0092] In this equation, μ_(P) is the friction coefficient between thebrake disk and pad, B is the effective radius of the pad, P_(W) is thebrake oil pressure.

[0093] Since I and r can be regarded as constants,$\mu \approx \frac{{{\omega}/{t}} + {TB}}{N}$

[0094] and this μ is used.

[0095] And dω/dt is detected by the wheel speed sensor now in use, N ismeasured by a vertical load sensor mounted on the suspension, and T_(B)is calculated by the above equation. In addition, if the knowneliminating means is used, since the need for considering the crosstalkwith respect to the output from the F sensor measured as a sensorcomponent T_(B) by the μ sensor is decreased, μ can be calculated bythis equation. The proper range around this maximum value is taken asthe optimum control start point.

[0096]FIG. 2 is a block diagram showing an example of a hard arrangementfor a control method for an anti-lock brake system for vehicles usingthe road surface friction coefficient μ according to the presentinvention described in claim 3, and this embodiment differs from thepreceding one in that instead of the ω, {dot over (ω)} detecting deviceA shown in FIG. 1, use is made of a {dot over (V)} detecting device A1using an acceleration sensor, the rest of the arrangement being the sameso that a repetitive description thereof is omitted. In this embodiment,to obtain the optimum control start point, the acceleration sensor isused and as for μ according to dV/dt, from the equation of wheel motion,

m·dV/dt=−μN

[0097] In the above equation, m is the vehicle weight, N is the wheelload, and μ is the friction coefficient. However, m can be regarded as aconstant.

[0098] From the above equation,

μ=−m dV/dt /N

[0099] and the μ in this equation is used.

[0100] And dV/dt is measured by the acceleration sensor and N ismeasured and calculated by the vertical load sensor mounted on thesuspension, and the proper range around this maximum value is taken asthe optimum control start point.

[0101]FIG. 3 is a block diagram showing an example of a hard arrangementfor a control method for anti-lock brake system for vehicles using theroad surface friction coefficient μ according to the present inventiondescribed in claim 4, which differs from the embodiments shown in FIGS.1 and 2 only in that an ω, {dot over (ω)}, {dot over (V)} detectingdevice A2 is used, the rest of the arrangement being the same so that arepetitive description thereof is omitted.

[0102]FIG. 4 is a block diagram showing an example of a hard arrangementfor a control method for an anti-lock brake system for vehicles usingthe road surface friction coefficient μ according to the presentinvention described in claims 5 and 6, and this embodiment differs fromthe embodiments shown in FIGS. 1 through 3 in that detecting devices A,A1, and A2 for (ω, {dot over (ω)}), ({dot over (V)}), and (ω, {dot over(ω)}, {dot over (V)}) respectively are selectively used and in that adiscriminating circuit B based on ω, {dot over (ω)}, {dot over (V)} isadded to the control device 3, the rest of the arrangement being thesame so that a repetitive description thereof is omitted.

[0103] The operation of the control device 3 in the embodiment shown inFIG. 4 will now be described on the basis of the flowcharts shown inFIGS. 5 through 8.

[0104] When the brake stepping-on force exceeds the preset value, theanti-lock brake device starts to operate, changing the normal brakeoperation to the anti-lock brake operation. The step 110 of the mainroutine shown in FIG. 5 represents the start of this anti-lock brakeoperation. Subsequently, at the step 111, the optimum control startpoint is discriminated on the basis of ω and {dot over (ω)} or {dot over(V)}, at the step 112, the road surface friction coefficient μ isdetected, and at the step 113, this value of μ is stored in a variablelabeled by μ t−1. Subsequently, at the step 114, this value is stored ina variable labeled by this μp. Then, after the brake liquid pressure isincreased at the step 115, μ is detected at the step 116. At the step117, the detected value of μ of the step 116 is stored in a variablelabeled by μt. Then the processing goes to the step 118, comparing thedifference μt−μt−1 between two values μt and μt−1 with a predeterminedreference value μc. If the difference μt−μt−1 is greater than μc, theprocessing goes to the step 119, and if it is equal to or smaller thanμc, the processing goes to the brake liquid pressure decreasing routineat the step 123. At the step 119, the value stored in the variable μt isstored in the variable μt−1 and this value of μt−1 is updated.subsequently the processing returns to the step 114.

[0105] At the brake liquid pressure decreasing routine 123, as shown inFIG. 6, first at the step 142 the brake liquid pressure is released ordecreased to a given lower level. Then at the step 143, the optimumcontrol start point is discriminated on the basis of ω and {dot over(ω)} or {dot over (V)} and after the μ is detected at the step 144, thisdetected value is stored in the variable at μt−1 at the step 145.

[0106] Then, the processing goes to the step 146, where it compares μt−1with a α·μp. The coefficient α is a constant preset at a suitableconstant value in the range of 0 to 1. If the variable at μt−1 issmaller, the processing goes to the step 149, where the brake liquidpressure decreasing routine 123 is completed, and it goes to the brakeliquid re-pressurizing routine at the step 124. If the variable μt−1 isgreater, it returns to the step 142.

[0107] At the brake liquid re-pressurizing routine 124 subsequent to thebrake liquid pressure decreasing routine 123, the processing shown inFIG. 7 is performed. First, at the step 162, the brake liquid pressureis increased. Subsequently, at the step 163, the optimum control startpoint is discriminated on the basis of ω, {dot over (ω)} or {dot over(V)}, and μ is detected at the step 164 and is stored in the variable μtat the step 165. Then, the variable μt is compared with the variableμt−1 at the step 166. If the variable μt is greater, the processing goesto the step 167 where the variable μt is stored in the variable μt−1 toupdate the stored value of the variable μt−1. Then, the processing goesto the step 171 to complete the brake liquid re-pressurizing routine,returning to the step 114 of the main routine. If the variable μt issmaller or equal at the step 166, the processing goes to the step 168 toupdate the variable μt−1 to the variable μt in the same manner as at thestep 167. And the processing returns to the step 162.

[0108] As the control device 3 performs the above-described processing,the anti-lick brake system according to the present invention operatesas follows. When the anti-lock brake system starts to operate, while therate of increase of the road surface friction coefficient μ exceeds apredetermined reference value, the brake liquid pressure is kept on theincrease and ω or dω/dt or dV/dt is used to decide the optimum controlstart point, and when the rate of increase of the road surface frictioncoefficient μ lowers below the reference value, the brake liquidpressure is lowered or released. At this time, after moving to thepressure decreasing mode, the control is moved from the pressuredecreasing mode to the pressure increasing mode by using the thresholdvalue for pressure decrease, the preset value of specified value ofelapsed time, or the discriminating circuit or a combination of thesefactors. Thereafter, the above operation is continue and repeated untilthe vehicle stops.

[0109] In the case where the vehicle speed lowers below a given valueduring the anti-lock brake operation, no matter which step in theflowchart shown in FIGS. 5 through 7 the control device 3 is performing,it immediately executes the interruption routine shown in FIG. 8 tocontrol the brake liquid pressure device such that it ends the anti-lockbrake operation and returns to the normal brake operation. If thevehicle speed is sufficiently low, there is no need for anti-lock brakeoperation and there is no need for it when the vehicle is stopping.

[0110] The above refers to an embodiment wherein the road surfacefriction coefficient μ is used, but in the case where the road surfacefriction force F is used, the same operation as in the above operationusing μ can be performed by multiplying μ by N from the relation

F=μ·N

[0111]FIG. 9 is a schematic flowchart showing a control method for ananti-lock brake system for vehicles using the road surface frictioncoefficient μ according to the present invention described in claim 7.If the driver steps on the hard brake in an emergency, the brakepressure is increased at the step 200. Then, at the step 202, whether ornot μ sharply increases is decided. If it sharply increases, it isdecided to be the hard brake and the ABS control starting at the step203 is performed. If μ decreases at the steps 204 and 205 irrespectiveof the brake pressure being on the increase, it is decided that the μmax value available on this road surface has been passed, and at thestep 206, the μ value before μ decreases is stored in MAX_μ_HI and theassociated brake pressure is stored in MAX_P_HI. At the step 207, on thebasis of this MAX value, the lower limit of vibration range of the usualroad surface is calculated as MAX_μ_LO and the corresponding brakepressure is calculated as MAX_P_LO. At the step 209, if the currentbrake pressure is greater than MAX_P_HI, the decrease of the brakepressure at the step 210 is effected, with the processing returning tothe step 208. Further, if the current brake pressure is less thanMAX_P_HI, the processing goes to the step 211. At the step 211, if thecurrent brake pressure is less than MAX_P_LO, the brake pressure isincreased at the step 212 and the processing returns to the step 208.Further, if, at the step 211, the current brake pressure is greater thanMAX_P_LO, the processing returns to the step 208 without doing anything.Thereby, the brake pressure can be maintained between μMAX and thecalculated fixed lower limit by a minimum of control.

[0112]FIG. 10 is a schematic flowchart showing a control method for ananti-lock brake system for vehicles using the road surface frictioncoefficient μ according to the present invention described in claim 8.The operation from the step 200 to the step 207 is exactly the same asin claim 7, so that a description thereof is omitted. In the loopincluding the steps 300, 301, 302, the pressure is decreased until thebrake pressure is equal to MAX_P_LO. Then, in the loop including thesteps 303, 304, 305, the pressure is increased until the brake pressureis equal to MAX_P_HI. Then, the processing returns to the loop includingthe steps 300, 301, 302. Thus, the brake pressure can be varied suchthat it is between the MAX value and the calculated fixed lower limit.

[0113]FIG. 11 is a schematic flowchart showing a control method for ananti-lock brake system for vehicles using the road surface frictioncoefficient μ according to the present invention described in claim 9.The operation from the step 200 to the step 305 is exactly the same asin claim 8, so that a description thereof is omitted. When theprocessing has gone through the loop including the steps 303, 304, 305,that is, if at the step 400 the current μ value is greater than MAX_μ_HIstored at the step 206, it can be decided that the vehicle has moved toa road surface where greater μ can be obtained. The processing returnsto the step 203 where the ABS control was started, and it is possible toseek the μMAX value available from this road surface.

[0114]FIG. 12 is a schematic flowchart showing a control method for ananti-lock brake system for vehicles using the road surface frictioncoefficient μ according to the present invention described in claim 10.The operation from the step 200 to the step 302 and from the step 303 tothe step 305 is exactly the same as in claim 8, so that a descriptionthereof is omitted. At the time of the step 500, the brake pressure isat the value of MAX_P_LO, and at the time of the step 207, thecorresponding μ was MAX_μ_LO. If the current μ is smaller than the valueof MAX_μ_LO, it is decided that the μ of the road surface has decreased,and after quick decrease of pressure at the step 502, the processingreturns to the ABS control start point at the step 203. At the time ofthe step 501, the brake pressure is at the value of MAX_P_HI, and thecorresponding μ at the time of the step 207 was at the value ofMAX_μ_HI. If the current μ is smaller than the value of MAX_μ_HI, it isdecided that the μ of the road surface has decreased, and after quickdecrease of pressure, the processing returns to the ABS control startpoint at the step 502.

[0115]FIG. 13 is a schematic flowchart showing a control method for ananti-lock brake system for vehicles using the road surface frictionforce F according to the present invention described in claim 7, itbeing noted that the road surface friction coefficient μ has beenreplaced by the road surface friction force F. Since the method ofcontrol can be performed in exactly the same way as in the case of μ, adetailed description thereof is omitted.

[0116] Further, the control using μ shown in FIGS. 10 through 12 can bereplaced by the control using F as in the case of FIG. 13.

Method of Detecting Control Point in ABS

[0117] The invention will now be described with reference to embodimentsshown in the drawings. FIG. 14 is a functional system diagram and FIG.15 is a typical stress sensor versus output graph obtained when avehicle is braked hard leading to the locking of the wheel. As can beseen from FIG. 15, when a vehicle is braked hard, for some time (afterbrake on) with sufficient friction force remaining on the road surface,as the curve P (brake oil pressure P) rises, the curve F (road surfacefriction force F) and curve T (brake torque T) increase at the same rateof increase in proportion to the curve P. It is known that theproportion value (F/T) in the output values of the curves F and T inthis period of time is constant. However, when the friction force(dependent on the tire and road conditions) obtained from the roadsurface approaches the limit, the brake torque represented by the curveT continues rising as usual in proportion to the brake oil pressure, butit is known that the rate of increase of the road surface friction forcerepresented by the curve F decreases and then rapidly decreases as soonas the brake oil pressure exceeds a given pressure (the brake forcecorresponding to the limit value of friction force obtained from theroad surface). Therefore, in this period of time, the proportion value(F/T) in the output values of the curves F and T rapidly decreases.

[0118] Further, it sometimes happens that the value of the brake torqueT is much greater than the road surface friction force F and with theroad surface friction force detecting means 1 of FIG. 14 it would bevery difficult to detect the pure road surface friction force F.Therefore, usually it follows that with the road surface friction forcedetecting means 1, a substantial amount of brake torque T mixes in andis measured as crosstalk. If, however, the proportion value (F/T) isused for decision in control rather than using the detected value of theroad surface friction force F, then

(F+t)/=F/T+t/T

[0119] where t is the crosstalk component which has mixed in the roadsurface friction force F. Further, since t is a value proportional tothe brake torque T, t/T is a constant. Therefore, in the case where F/Tis used to detect the change therein so as to decide the control point,it is possible to effect control of the type in which crosstalkcomponent due to the brake torque T is eliminated.

[0120] Thus, as shown in FIG. 14, immediately after braking, thedetected value from the road surface friction force detecting device 1and the detected value from the brake torque detecting means 2 are fedto the arithmetic means 3, where the F/T is successively calculated andthe result is fed to the decision means 4. In the decision means 4, theratio F/T is monitored immediately after the braking and at the point oftime when this ratio has suddenly decreased (the change has increased),it is decided that the wheel s going to be locked. And this point oftime is detected as the point of time for the first pressure decreasingtiming in ABS control and the detected signal is used to give a brakeoil pressure decreasing instruction to the ABS control device 5; in thismanner, the ABS control to avoid the locking of the wheel is madepossible.

[0121] If the brake oil pressure P is increased (the oil ispressurized), the brake torque T increases through the transmissiondelay in the brake system, and through the transmission delay the roadsurface friction force F increases. Generally, the region on theleft-hand side of the peak of the curve F in FIG. 15 is called thestable region, while the region on the right-hand side of the peak ofthe curve F in FIG. 15 is called the unstable region. If the balance ismaintained in the stable region, even if the road surface friction forceF acting between a tire and the road surface changes owing to sendingsmall stones flying, a force acts by which the original position can beinstantly restored. However, if said change in the road surface frictionforce F takes place in the unstable region, it rapidly moves in thewheel locking direction (to the right-hand side of the peak of the curveF) or to the position in the stable region where balance can bemaintained.

[0122] Therefore, in claim 15, the ratio F/T being constant (the firstdifferential of F/T being 0) means stability in the particular position(to the left-hand side of the peak of the curve F) in the graph of FIG.15, which means that the brake oil pressure P and the brake torque T arebalanced with each other. The ratio F/T being constant (the firstdifferential of F/T being 0) means that for example, the brake oilpressure decreasing control when the brake oil pressure P is excessivelyincreased and moves to the unstable region moves it to the stable regionto provide the road surface friction force F which is proportional tothe current brake oil pressure P. This serves as a confirmation decisionallowing the next control to be performed upon completion of thepreceding control.

[0123] The time when the brake oil pressure P is being decreased is thetime when it goes too far in the direction of unstability to result inthe F/T value being high, and the decision of stopping decreasing thebrake oil pressure can better be performed by referring to the precedingor first successful control. In the case of referring to the precedingcontrol, if the brake oil pressure P or the brake torque T which isproportional to P before the locking of the wheel is used to decide thepressure decrease stopping point, the optimum control oil pressure P maychange to result in erroneous control if there is a change of the roadsurface between the preceding time of control and this time of control(for example, from a dry asphalt pavement to a road wet with water).However, if the F/T value is used for decision, since the F/T value doesnot almost change, the decision using the comparison with the precedingvalue described in claim 16 or the comparison with the first controlvalue described in claim 17 results in less erroneous decision and hencesafer control decision; thus, such means is very effective.

[0124] As for the decision of stopping decrease of the brake oilpressure, if the delay in the control system is greater than theresponse required by the control, fruitless pressure decrease would beperformed to lose the braking distance unless the control is effectedbefore the F/T value fully decreases to the target value. This timealso, as in the above, the road surface friction force F is directlyused and in order not to result in erroneous control owing to a changeof the road surface, decision is made by at the point where the F/Tvalue starts to move in the target direction (the point where it startsto decrease) by using the F/T value as described in claim 18; in thismanner, safer control can be effected.

[0125] In the case where the F/T value increases during the retaining ofthe brake oil pressure P or during pressure control, this means that thepressure is moving in the wheel locking direction (to the right in thegraph of FIG. 15); therefore, the control is moved to decrease the brakeoil pressure P. This is not almost influenced by a change of the roadsurface since the F/T value is used as described in claim 19,; thus,erroneous control can be effectively eliminated.

[0126] In the brake oil pressure retaining control, if the F/T isgreater than the value obtained in the first or preceding brake pressuredecreasing control is stable, this means that the preceding controldecision value is too small, so that mild pressure control is effected(claim 20) and is the F/T value in on the decrease, it is decided thatit is moving in the unstable direction (to the right-hand side of FIG.15), so that the pressure decreasing control is effected (claim 21). Ifthe F/T value is small and stable, this means that the friction force Favailable from the road surface is on the increase, so that the mildpressure control is effected (claim 22).

[0127] In claim 23, as in the approximate equation of motion shown below

I·dω/dt=k1·F−k2·T

[0128] from the moment of inertia I of the wheel and F and T multipliedby the proportionality constants k1 and k2, respectively, the F−T valueis a value proportional to the wheel acceleration dω/dt. This can beused as a wheel speed sensor; thus, there has been realized a wheelspeed sensor capable of real time sensing which has eliminated thedrawback of being lacking in response (the slower the sensing, the worsethis situation) which is characteristic of the conventional gear-likewheel speed sensor. Hybrid control using both the slip ratio controlbased on the wheel speed according to the prior art and the controldecision based on F or μ according to the present invention are madepossible by a single sensor.

[0129] In all embodiments described above, F/T has been used, butsubstantially the same result can also be obtained by using μ/T.However, since μ=F/T, the load moving component of the vehicle isincluded in μ.

What is claimed is:
 1. A method of controlling an anti-lock brake systemfor vehicles wherein when a vehicle having an anti-lock brake systemmounted thereon has emergency brake applied thereto, a change in roadsurface friction force or road surface friction coefficient due to theincreased brake pressure is detected by a road surface friction forcedetecting device or a road surface friction coefficient detectingdevice, said method being characterized in that to decide the optimumcontrol start point concomitant with the increase of the signal value ofroad surface friction force F or road surface friction coefficient μdetected by the road surface friction force detecting device or roadsurface friction coefficient detecting device, use is made of means foreliminating disturbance sources which impede such decision.
 2. A methodof controlling an anti-lock brake system for vehicles, wherein when avehicle having an anti-lock brake system mounted thereon has emergencybrake applied thereto, a change in road surface friction force or roadsurface friction coefficient due to the increased brake pressure isdetected by a road surface friction force detecting device or a roadsurface friction coefficient detecting device, said method beingcharacterized in that the optimum control start point concomitant withthe increase of the signal value of road surface friction force F orroad surface friction coefficient μ due to the increased brake pressuredetected by the road surface friction force detecting device or roadsurface friction coefficient detecting device is decided by a drop inthe wheel speed, that is, by using the wheel speed ω or dω/dt, and thenfrom the point where the specified value of control by F or μ or bydF/dt or dμ/dt is reached, the brake pressure is shifted from thepressure increasing mode to the pressure retaining or decreasing mode,so as to eliminate disturbance sources.
 3. A method of controlling ananti-lock brake system for vehicles wherein when a vehicle having ananti-lock brake system mounted thereon has emergency brake appliedthereto, a change in road surface friction force or road surfacefriction coefficient due to the increased brake pressure is detected bya road surface friction force detecting device or a road surfacefriction coefficient detecting device, said method being characterizedin that the optimum control start point concomitant with the increase ofthe signal value of road surface friction force F or road surfacefriction coefficient μ due to the increased brake pressure detected bythe road surface friction force detecting device or road surfacefriction coefficient detecting device is decided by a drop in thevehicle acceleration decided by dV/dt using an acceleration sensor, andthen from the point where the specified value of control by F or μ or bydF/dt or dμ/dt is reached, the brake pressure is shifted from thepressure increasing mode to the pressure retaining or decreasing mode,so as to eliminate disturbance sources.
 4. A method of controlling ananti-lock brake system for vehicles wherein when a vehicle having ananti-lock brake system mounted thereon has emergency brake appliedthereto, a change in road surface friction force or road surfacefriction coefficient due to the increased brake pressure is detected bya road surface friction force detecting device or a road surfacefriction coefficient detecting device, said method being characterizedin that the optimum control start point concomitant with the increase ofthe signal value of road surface friction force F or road surfacefriction coefficient μ detected by the road surface friction forcedetecting device or road surface friction coefficient detecting deviceis decided by both the wheel speed ω or dω/dt and dV/dt, and then fromthe point where the specified value of control by F or μ or by dF/dt ordμ/dt is reached, the brake pressure is shifted from the pressureincreasing mode to the pressure retaining or decreasing mode, so as toeliminate disturbance sources.
 5. A method of controlling an anti-lockbrake system for vehicles as set forth in claims 1 through 4,characterized in that in the pressure decreasing mode subsequent to theretaining mode or after moving to the pressure decreasing mode, thebrake pressure is shifted from the pressure decreasing mode to thepressure increasing mode by a pressure decreasing threshold value, a setvalue of a specified value of elapsed time or a decision circuit or by acombination of two or more of such factors, so as to eliminatedisturbance sources.
 6. A method of controlling an anti-lock brakesystem for vehicles, characterized in that during travel of a vehiclehaving an anti-lock brake system, the first brake pressure control iseffected by using any of the control methods described in claims 1through 4, and then at the point of time when the control methoddescribed in claim 5 is completed, the control is continuouslyrepetitively effected by using any of the control methods described inclaims 1 through 4 or the control method described in claim 5 until thevehicle stops or in and after the second time the control iscontinuously repetitively effected by using F or μ, or dF/dt or dμ/dtalone and the control method described in claim 5 until the vehiclestops, thus eliminating disturbance sources.
 7. A method of controllingan anti-lock brake system for vehicles wherein when a vehicle having ananti-lock brake system mounted thereon has emergency brake appliedthereto, a change in road surface friction force or road surfacefriction coefficient due to the increased brake pressure is detected bya road surface friction force detecting device or a road surfacefriction coefficient detecting device, said method being characterizedin that after the maximum value of the signal value of road surfacefriction force F or road surface friction coefficient μ detected by theroad surface friction detecting device or road surface frictioncoefficient detecting device has been ascertained, fixed lower limitsare provided for the approximate maximum F value or approximate maximumμ value and the maximum F value or maximum μ value and the brakepressure is controlled such that F or μ is stably retained in the range,thus eliminating disturbance sources.
 8. A method of controlling ananti-lock brake system for vehicles wherein when a vehicle having ananti-lock brake system mounted thereon has emergency brake appliedthereto, a change in road surface friction force or road surfacefriction coefficient due to the increased brake pressure is detected bya road surface friction force detecting device or a road surfacefriction coefficient detecting device, said method being characterizedin that after the maximum value of the signal value of road surfacefriction force F or road surface friction coefficient μ detected by theroad surface friction detecting device or road surface frictioncoefficient detecting device has been ascertained, fixed lower limitsare provided for the approximate maximum F value or approximate maximumμ value and the maximum F value or maximum μ value and the brakepressure is controlled by being increased or decreased in the range inwhich F or μ is retained, thus eliminating disturbance sources.
 9. Amethod of controlling an anti-lock brake system for vehicles,characterized In that in claims 7 and 8, variations in F or μ withrespect to brake pressure are monitored and when the upper limit ofbrake pressure in a preset control range is reached, if F or μ exceedsthe previous maximum F or maximum μ, the brake pressure is furtherincreased to ascertain the maximum μ again, whereafter theabove-mentioned brake pressure control is effected, thus eliminatingdisturbance sources.
 10. A method of controlling an anti-lock brakesystem for vehicles, characterized in that in claims 7 and 8, variationsin F or μ with respect to brake pressure are monitored and if the Fvalue or μ value decreases with respect to the brake pressure in apreset brake pressure control range, the brake pressure is rapidlydecreased to ascertain the maximum μ again, thus effecting control toeliminate disturbance sources.
 11. A method of controlling an anti-lockbrake system for vehicles, characterized in that in the case where themaximum value of the signal value of road surface friction force F orroad surface friction coefficient μ detected by the road surfacefriction force detecting device or road surface friction coefficientdetecting device can hardly be ascertained, quasi-F or quasi-μ is foundbetween the maximum F value or maximum μ value and the minimum F valueor minimum μ value within a given period of time, and the brake pressurecontrol is effected corresponding to such quasi-F or quasi-μ, thuseliminating disturbance sources.
 12. A method of controlling ananti-lock brake system for vehicles, characterized in that in claim 11 ,in the case where the value between the maximum F value or maximum μvalue and the minimum F value or minimum μ value within a given periodof time varies beyond the allowable range, the individual values aremeasured again within a given period of time to newly find quasi-F orquasi-μ and the brake pressure control is effected corresponding to suchquasi-F or quasi-μ, thus eliminating disturbance sources.
 13. A methodof controlling an anti-lock brake system for vehicles, characterized inthat in claims 11 and 12, in the case where the maximum μ can beascertained, control is effected according to the control methodsdescribed in claims 7 through 10, thus eliminating disturbance sources.14. A method of detecting the control point in an ABS having a stresssensor which provides an output proportional to the road surfacefriction F or road surface friction coefficient μ having mixed therein acrosstalk component, such as brake torque T, and to the brake torque T,said method being characterized in that it uses adjusting means formaking adjustment from the rise of the brake start such that detectedsignals of F or μ and T are adjusted in ratio or made equal in value,and decision means for deciding the control point by a change in saidratio or in adjustment coefficient, wherein the timing of the controlpoint is detected by the size of a change in the ration of detectedsignals of F or μ and T or in adjustment coefficient.
 15. A method ofdetecting the control point in an ABS, characterized in that in thedecision means of claim 14 , the point of time when the ratio of F or μand T or adjustment coefficient substantially stops changing is decidedto be the control timing.
 16. A method of detecting the control point inan ABS, characterized in that in the decision means of claim 14 , thepoint of time when the ratio of F or μ and T or the adjustmentcoefficient, during brake pressure decreasing control, becomessubstantially equal to the value obtained upon completion of thepreceding brake pressure decreasing control is decided as the completionof the brake pressure decreasing control, said point of time beingdecided to be the optimum control timing for ABS.
 17. A method ofdetecting the control point in an ABS, characterized in that in thedecision means of claim 14 , the point of time when the ratio of F or μand T or the adjustment coefficient, during the brake pressuredecreasing control, becomes above the value obtained during the firstbrake pressure decreasing control is decided to be the completion of thebrake pressure decreasing control, said point of time being decided tobe the optimum control timing for the ABS.
 18. A method of detecting thecontrol point in an ABS, characterized in that in the decision means ofclaim 14 , the point of time when the ratio of F or μ and T or theadjustment coefficient, during the brake pressure decreasing control,starts to increase is decided to be the completion of the brake pressuredecreasing control, said point of time being decided to be the optimumcontrol timing for the ABS.
 19. A method of detecting the control pointin an ABS, characterized in that in the decision means of claim 14 , thepoint of time when the ratio of F or μ and T or the adjustmentcoefficient, except during the brake pressure decreasing control,becomes just above the value obtained during the first brake pressuredecreasing control is decided as the start of brake pressure decreasingcontrol, said point of time being decided to be the optimum controltiming for the ABS.
 20. A method of detecting the control point in anABS, characterized in that in the decision means of claim 14 , the pointof time when the ratio of F or μ and T or the adjustment coefficient,during the brake pressure retaining control, becomes greater than thevalue obtained during the first brake pressure decreasing control isdecided to be the start of brake pressurization control, said pointbeing decided to be the optimum control timing for the ABS.
 21. A methodof detecting the control point in an ABS, characterized in that in thedecision means of claim 14 , the point of time when the ratio of F or μand T or the adjustment coefficient, during the brake pressure retainingcontrol, becomes smaller than the value obtained during the first brakepressure decreasing control and becomes further smaller is decided to bethe start of brake pressure decreasing control, said point being decidedto be the optimum control timing for the ABS.
 22. A method of detectingthe control point in an ABS, characterized in that in the decision meansof claim 14 , the point of time when the ratio of F or μ and T or theadjustment coefficient, during the brake pressure retaining control,becomes smaller than the value obtained during the first brake pressuredecreasing control and is stabilized is decided to be continuation ofbrake pressure retention or the start of control of gentle increase ofpressure, said point of time being decided to be the optimum controltiming for the ABS.
 23. A method of detecting the control point in anABS having a stress sensor which provides an output proportional to theroad surface friction F or road surface friction coefficient μ havingmixed therein a crosstalk component, such as brake torque T, and to thebrake torque T, said method being characterized in that it usesadjusting means for adjusting the ratio of detected signals of F or μand T, and decision means for deciding the control point by a change inF−T value from its value after adjustment, wherein the timing for thecontrol point is detected by said change in the F−T value.
 24. A methodof detecting the control point in an ABS having a stress sensor whichprovides an output proportional to the road surface friction force F orroad surface friction coefficient μ and the brake torque T, said methodbeing characterized in that the timing for the control point is detectedby a change in the F−T value during brake control.
 25. A method ofdetecting the control point in an ABS, characterized in that in thedecision means in claims 23 and 24, a threshold value for the F−T valueis provided to decide the control point in control.
 26. A method ofdetecting the control point in an ABS, characterized in that in thethreshold value of claim 25 , a change in the F−T threshold value iscorrected as the road surface friction coefficient changes, therebydetecting the timing for the control point.